The present invention relates to wireless sensors, and more particularly to systems and methods for remotely powering and remotely determining the value of sensor in a wireless remote sensor.
There is a growing focus on and the development of wireless remote sensors. Wireless remote sensors may be used in a variety of applications. For example, wireless remote sensors may include transdermal and internal medical sensors, inductively powered heating and cooking containers and sensors located in product packaging. Some exemplary wireless remote sensors are shown in U.S. patent application Ser. No. 13/082,503, entitled “Point of Sale Inductive Systems and Methods,” filed Apr. 8, 2011, by Baarman et al and U.S. patent application Ser. No. 13/082,513, entitled “Point of Sale Inductive Systems and Methods,” filed Apr. 8, 2011, by Baarman et al, both of which are incorporated herein by reference in their entirety.
Wireless remote sensors are convenient in that they can provide a mechanism for measuring relevant information and communicating that measurement to a receiver without the need for wires. For examples, measurements can be taken with plugging in the sensor. Further, after the measurement is taken, it is not necessary to unplug the sensor. With internal medical sensors the importance of wireless communication with the sensor is even greater.
Wireless remote sensors are typically powered by and communicate with an inductive transceiver. This means that the power used to operate the sensor can be provided to the wireless remote sensor without the need for wires or other direct electrical contact. For example, a number of conventional remote sensors wirelessly receive power from an inductive wireless power supply. With inductively powered wireless remote sensors, it is not uncommon for the sensor value (e.g. the value being measured by the remote sensor) to be communicated to the inductive transmitter via reflected impedance. Many conventional wireless remote sensors incorporate a sensor having a capacitance or resistance that varies as a function of the parameter to be measured. For example, when measuring temperature, the sensor may include a thermistor having an impedance that varies with temperature. As another example, the sensor may include a capacitor with a dielectric that varies its dielectric coefficient over temperature. The remote sensor is typically configured with the variable capacitor or resistor arranged as part of a simple RLC circuit so that variations in the sensor result in changes in the resonant frequency of the RLC circuit. The RLC circuit is inductively coupled to the inductive transmitter so that the RLC circuit affects a characteristic of power in the inductive transmitter via reflected impedance. For example, the reflected impedance of the RLC circuit can affect the amplitude of current in the inductive transmitter tank circuit. Accordingly, in use, the value of the sensor is communicated back to the inductive transmitter, where it can be detected by a sensor that monitors a characteristic of power in the inductive transmitter, for example, in the amplitude of current and/or the resonant frequency of power in the inductive transmitter tank circuit.
Experience has revealed that relatively small variations in the impedance of a wireless remote sensor are difficult to detect in the inductive transmitter. Further, these variations can be masked by changes in the coupling between the inductive transmitter and the remote sensor or by variations in manufacturing tolerances.